1. Field of the Invention
The present invention relates to a fixing apparatus for an electronic induction heating system and an image forming apparatus equipped with the fixing apparatus, such as a copying machine, facsimile, or printer which utilize an electrophotography device or an electrostatic recording format. The present invention further relates to a wire winding apparatus and a method for producing a magnetic excitation coil that is utilized for an electronic induction heating system, using the wire winding apparatus.
2. Description of Related Art
Generally, fixing devices installed in an image forming apparatus such as a printer or a copying machine are designed to save energy and operate at high speeds. Because of this, devices equipped with an electromagnetic induction heating type heater can be widely utilized in place of halogen lamps and similar devices. Related electromagnetic induction heating type heaters apply a magnetic field, generated by a magnetic excitation coil, to a heating element which then heats the heating element. As an example, this heater can be used as a fixing device that heats non-fixed images formed on a recording medium such as transfer paper or an OHP sheet.
Related heaters equipped with magnetic excitation coils apply a magnetic field, generated by the magnetic excitation coil, to a cylindrical heat generation roller which in turn generates an eddy current in an electroconductive layer formed by the surface layer of the heat generation roller. The electroconductive layer is then heated by the Joule heat generated by this eddy current. Normally, it is effective to design the shape of the magnetic excitation coil along the outside shape of the cylindrical heat generation roller. Fixing devices which are equipped with heat-resistant endless belts suspended between a fixing roller, a heat generation roller, and both rollers are well known. It is also preferable to closely place a magnetic excitation coil along the heat generation roller and endless belt in a fixing device equipped with this type of endless belt.
In order to stably maintain an approximate cylindrical shape when forming a magnetic excitation coil into an approximate cylindrical shape, from the past, the magnetic excitation coil was changed into the desired shape (semicircular shape) along with a coil shape retaining member (as an example refer to related art 1). Further, when forming the magnetic excitation coil into an approximate cylindrical shape, there is a chance that the temperature distribution of the heating element may not be uniform. In order to make the temperature distribution of the heating element uniform, it has been proposed to partially change the distance between the magnetic excitation coil and the heating element to partially alter the magnetic excitation coil making it more distant from the heating element (as an example refer to related art 2).                [Related Art 1] Japanese Patent Publication 2000-243545        [Related Art 2] Japanese Patent Publication H9-26719        
If the diameter of the heat generation roller is made smaller than the fixing roller in a fixing device in which an endless heat-resistant belt is suspended between the fixing roller and the heat generation roller, the endless heat-resistant belt will be expanded into a fan shape towards the fixing roller. In addition, there is a chance that the heat-resistant belt might bulge close to the contact area between the heat generation roller following the rotation of the belt.
Even if the magnetic excitation coil formed in an approximate semicircular shape matching the curvature of the heat generation roller is placed close to the heat generation roller, a problem occurs wherein the edge of the open side of the magnetic excitation coil interferes with the heat-resistant belt that is expanded into a fan shape or the heat-resistant belt that is bulging. In order to avoid interference between the magnetic excitation coil and the heat-resistant belt, the installation range of the magnetic excitation coil can be restricted to contact area L between the heat generation roller and the heat-resistant belt although this does not act in response to the requirement to provide efficient heating up to a range wider than contact area L by closely placing the magnetic excitation coil. Separating the distance between the heat-resistant belt and the magnetic excitation coil enough that they do not interfere with each other in order to avoid interference between the magnetic excitation coil and the heat-resistant belt even further results in a problem of a worsening magnetic bonding between the magnetic excitation coil and the heat generation roller and heat generation loss.
In related art 1, after securing a planar coil shape retaining member on a planar coil winding jig and winding the coil wire on the planar surface of the above-mentioned coil shape retaining member, the coil is pressurized. Then, a substantially planar-shaped coil is heated until reaching a softening temperature together with the coil shape retaining member and changed into a shape that forms the surface of the coil.
When changing the pressurized planar-shaped coil as required together with the coil shape retaining member, the outside applies stress in a direction that compresses the inside towards a direction that stretches the copper wire towards the bending direction of the magnetic excitation coil copper wire wound in a substantially planar shape. As a result, a problem of stress being applied to the insulation cover of the copper wire and the insulation cover deteriorating occurred. In particular, the magnetic excitation coil copper wire used in an induction heater had a remarkable problem because Litz wire was used for this copper wire and this Litz wire used a very thin cooper wire wound several tens of times.
As disclosed in related art 2, if the bending amount of the magnetic excitation coil is utilized to make the heat generation distribution uniform, there as also a problem of more stress being applied to the coil and the stress applied to the insulation cover of the copper wire increasing even more.